a) Field of the Invention
The present invention relates to an optical semiconductor module having an optical semiconductor element which is resin-sealed to improve moisture resistance, its manufacture method, a reflection film, its manufacture method, and laser and optical devices using reflection films.
b) Description of the Related Art
With the advent of multimedia societies, subscriber networks are now vigorously changed to optical networks. In order to prevail optical fibers to subscriber networks, it is necessary to lower the cost of optical components, particularly optical semiconductor modules which have a large influence upon the total cost.
Conventional low price optical semiconductor modules have been manufactured by a simple coupling method of coupling an optical semiconductor element and an optical waveguide such as an optical fiber on an Si platform or by a simple sealing method of sealing an optical semiconductor element by directly potting resin. These methods can reduce the number of components and lower the cost. Conventional solder sealing has been replaced by cheap resin sealing to lower the cost required for protecting an optical semiconductor element from external moisture.
Epoxy resin transparent to reception/emission light has been used as sealing resin. The publication of JP-A-HEI-8-18163 discloses a double sealing structure which covers an optical semiconductor element with silicon resin having rubber elasticity and also covers the silicon resin coated the optical semiconductor element with epoxy resin which shields sun light and water contents.
Although epoxy resin has a high moisture resistance, it has a large coefficient of linear expansion so that the optical semiconductor element may be damaged by a resin volume change to be caused by a temperature change. When LSI using Si or other components is resin-sealed, epoxy resin is mixed with filler such as silica in order to relax thermal stress to be applied to the semiconductor element. However, when an optical semiconductor element is resin-sealed, the resin cannot be mixed with filler because it is necessary to provide optical coupling between the optical semiconductor element and an optical fiber.
If silicon resin is used as the sealing resin, thermal stress applied to an optical semiconductor element can be relaxed because it has rubber elasticity. However, since silicon resin has higher moisture permeability than epoxy resin, it is difficult to ensure a sufficient moisture resistance of an optical semiconductor module.
Double sealing with silicon resin and epoxy resin can relax thermal stress while the moisture resistance is retained. It is, however, necessary to perform at least two resin curing processes because the curing conditions of two types of resin are different. This increases the number of manufacture processes, and contradicts the demands of low cost. An insufficient strength of uncured resin is likely to occur.
Next, issues regarding resin sealing will be described by paying attention to an optical viewpoint.
A reflection film of a multi-layer structure is know which is a lamination of two or more thin films having an optical film thickness of a quarter wavelength of light to be reflected. If this reflection film of the multi-layer structure is formed on the facets of an optical resonator of a laser diode, the laser diode can have a low threshold current, a high output, and the like.
The fundamental parameters as indices of the laser characteristics of a semiconductor laser device having a pair of resonator facets include a threshold gain, an external differential quantum efficiency, a front/back ratio, and a slope efficiency. The threshold gain is defined by:
gth=xcex1i+(1/L)ln(1/(RfRr)xc2xd)
where xcex1i is an internal loss of an optical resonator, L is a resonator length, Rf and Rr are reflectivities at the front and back facets.
The external differential quantum efficiency xcex7d is defined by:
xcex7d=xcex7ixc3x97ln(1/R)/(xcex1iL+ln(1/R))
where xcex7i is an internal quantum efficiency and an assumption of R=Rf=Rr is incorporated.
The front/back ratio r is defined by:
r=((1xe2x88x92Rf)/(1xe2x88x92Rr))xc3x97(Rr/Rf)xc2xd.
The slope efficiency Sd is defined by:
Sd=1.24xc3x97xcex7d/xcex
where xcex is an oscillation wavelength.
As seen from the above definition equations, as the reflectivities Rf and Rr lower, the threshold gain gth lowers although the external differential quantum efficiency xcex7d and slope efficiency Sd become high. Namely, the threshold current increases. An increase of the threshold current may degrade the optical output characteristics, particularly under the high temperature operation environment.
Evaluation of the laser diode characteristics is generally performed in the atmospheric air or in an inert gas atmosphere. In actual operation, a laser diode is mounted on a substrate and thereafter covered with resin or the like. As the reflection facet of an optical resonator is covered with resin, the reflectivity lowers and the threshold gain gth increases. Therefore, it is difficult to evaluate the optical output characteristics under the actual operation conditions.
It is an object of the present invention to provide an optical semiconductor module having a sufficient moisture resistance and is easy to be made low in cost, and its manufacture method.
It is another object of the present invention to provide a reflection film, its manufacture method, and a semiconductor laser device, capable of evaluating the characteristics of the semiconductor laser device under the conditions approximate to the actual operation conditions.
According to one aspect of the present invention, there is provided an optical semiconductor module comprising: a platform having a support surface; an optical semiconductor element disposed on the support surface of the platform; an optical element disposed on the support surface of the platform and having an optical coupling facet, the optical element being optically coupled to the optical semiconductor element at the optical coupling facet; and a protective member made of gel acrylic modification resin, covering the optical semiconductor element, and being disposed at least in a light transmission area in a space between the semiconductor element and the optical coupling facet of the optical element.
Since the protective member is made of gel resin, a thermal stress applied to the optical semiconductor element can be reduced. Since the acrylic modification resin has a low moisture permeability and moisture absorption, it is possible to prevent water contents from invading into the optical semiconductor element.
According to another aspect of the present invention, there is provided a method of manufacturing an optical semiconductor module comprising the steps of: disposing an optical semiconductor element and an optical element on a support surface of a platform so as to optically coupling the optical semiconductor device and the optical element; placing the platform on an inner surface of an outer frame, with ultraviolet curing type adhesive being interposed between the platform and the inner surface; disposing an acrylic modification resin composition covering the semiconductor element on the platform and being filled in a light transmission area between the optical semiconductor element and the optical element, the acrylic modification resin composition being cured upon radiation of ultraviolet rays and becoming gel; and curing the adhesive and the acrylic modification resin composition by radiating ultraviolet rays.
Both the adhesive and acrylic modification resin composition can be cured at the same time by one ultraviolet ray radiation process, so that the number of manufacture processes can be reduced.
According to another aspect of the present invention, there is provided a method of manufacturing a reflection film comprising the step of preparing an optical medium having a reflection surface and refractive index n0; determining a wavelength xcex of light to be reflected; determining two different reference refraction indices ns1 and Ns2; laminating k (k is a positive integer) pairs of a first layer having a refractive index n1 and a second layer having a refractive index n2 on the reflection surface of the optical medium, wherein a thickness of the first layer is:
(xcex/4+(xcex/2)xc3x97N1)/n1
where N1 is 0 or a positive integer, and a thickness of the second layer is:
(xcex/4+(xcex/2)xc3x97N2)/n2
where N2 is 0 or a positive integer; determining a thickness d3 of a third layer having the refractive index n1 which satisfies:
d3=d+(xcex/2n1)xc3x97N3
where N3 is 0 or a positive integer; and                     cos        2            ⁢      Δ        =                                                      n              1              2                                      n              ⁢                              xe2x80x83                            ⁢                              s                1                            ⁢              n              ⁢                              xe2x80x83                            ⁢                              s                2                                              ⁢                      n            1            2                    ⁢                      a            4                          -                  n          0          2                                      (                      1            +                                          n                1                2                                            n                ⁢                                  xe2x80x83                                ⁢                                  s                  1                                ⁢                n                ⁢                                  xe2x80x83                                ⁢                                  s                  2                                                              )                ⁢                  (                                                    n                1                2                            ⁢                              a                4                                      -                          n              0              2                                )                                Δ      =                        2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      n            1                    ⁢                      d            1                          λ              ,          a      =                        (                                    n              1                                      n              2                                )                k            
(k is 0 or a positive integer);
and forming the third film having the thickness d3 determined at the step of determining the thickness d3 on a surface of the second layer at the k-th pair.
The reflectivity of the reflection film in a medium having a refractive index ns1 is equal to the reflectivity of the reflection film in a medium having a refractive index ns2. Therefore, the optical characteristics of the optical component using this reflection film in the medium having the refractive index ns2 can be predicted by measuring the optical characteristics in the medium having the refractive index ns1.
According to another aspect of the present invention, there is provided a laser device comprising: a laser medium having an oscillation wavelength xcex and two reflection facets and defining an optical resonator; a lamination structure formed on at least one reflection facet of the optical resonator defined by the laser medium, the lamination structure being formed by laminating k (k is a positive integer) pairs of a first layer having a refractive index n1 and a second layer having a refractive index n2 wherein a thickness of the first layer is:
(xcex/4+(xcex/2)xc3x97N1)/n1
where N1 is 0 or a positive integer, and a thickness of the second layer is:
xe2x88x92(xcex/4+(xcex/2)xc3x97N2)/n2
where N2 is 0 or a positive integer; a third layer having the refractive index n1 formed on the second layer at the k-th pair of the lamination structure; and a protective member made of a material having a refractive index ns and covering a surface of the third layer, wherein if the thickness d3 of the third layer is:
d3=d+(xcex/2n1)xc3x97N3
where N3 is 0 or a positive integer, then:                     cos        2            ⁢      Δ        =                                                      n              1              2                                      n              ⁢                              xe2x80x83                            ⁢              s                                ⁢                      n            1            2                    ⁢                      a            4                          -                  n          0          2                                      (                      1            +                                          n                1                2                                            n                ⁢                                  xe2x80x83                                ⁢                s                                              )                ⁢                  (                                                    n                1                2                            ⁢                              a                4                                      -                          n              0              2                                )                                Δ      =                        2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      n            1                    ⁢          d                λ              ,          a      =                        (                                    n              1                                      n              2                                )                k            
(k is 0 or a positive integer.) is satisfied.
The reflectivity of the reflection film in a medium having a refractive index ns1 is equal to the reflectivity of the reflection film in a medium having a refractive index ns2. Therefore, the optical characteristics of the laser diode in the medium having the refractive index ns2 can be predicted by measuring the optical characteristics in the medium having the refractive index ns1.
According to another aspect of the present invention, there is provided an optical device comprising: an optical medium having a refractive index n0 and a defined reflection facet; a lamination structure formed on the reflection facet of the optical medium, the lamination structure being formed by laminating k (k is a positive integer) pairs of a first layer having a refractive index n1 and a second layer having a refractive index n2 wherein optical thicknesses of the first layer and the second layer relative to light having a wavelength xcex are both:
xcex/4+(xcex/2)xc3x97N
where N is 0 or a positive integer; a third layer having the refractive index n1 and formed on the second layer at the k-th pair of the lamination structure; and a protective film made of a material having a refractive index ns and covering a surface of the third layer, wherein if the thickness d3 of the third layer is:
d3=d+(xcex/2n1)xc3x97N
where N is 0 or a positive integer, then:                     cos        2            ⁢      Δ        =                                                      n              1              2                                      n              ⁢                              xe2x80x83                            ⁢              s                                ⁢                      n            1            2                    ⁢                      a            4                          -                  n          0          2                                      (                      1            +                                          n                1                2                                            n                ⁢                                  xe2x80x83                                ⁢                s                                              )                ⁢                  (                                                    n                1                2                            ⁢                              a                4                                      -                          n              0              2                                )                                Δ      =                        2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      n            1                    ⁢          d                λ              ,          a      =                        (                                    n              1                                      n              2                                )                k            
(k is 0 or a positive integer.) is satisfied.
The reflectivity of the reflection film constituted of the first to third layers in the atmospheric air is generally equal to that of the reflection film covered with the protective member. Therefore, the optical characteristics of the optical device after covered with the protective member can be predicted by measuring the optical characteristics measured in the atmospheric air before covering with the protective member.
According to another aspect of the present invention, there is provided an optical device comprising: an optical medium having a refractive index n0 and a defined reflection facet; a lamination structure formed on the reflection facet of the optical medium, the lamination structure being formed by laminating k (k is a positive integer) pairs of a first layer having a refractive index n1 and a second layer having a refractive index n2 wherein optical thicknesses of the first layer and the second layer relative to light having a wavelength xcex are both:
xcex/4+(xcex/2)xc3x97N
where N is 0 or a positive integer; and a third layer having the refractive index n1 and formed on the second layer at the k-th pair of the lamination structure, wherein if the thickness d3 of the third layer is:
d3=d+(xcex/2n1)xc3x97N
where N is 0 or a positive integer, then:                     arccos        (                                                                                                  n                    1                    2                                    16                                ⁢                                  n                  1                  2                                ⁢                                  a                  4                                            -                              n                0                2                                                                    (                                  1                  +                                                            n                      1                      2                                        16                                                  )                            ⁢                              (                                                                            n                      1                      2                                        ⁢                                          a                      4                                                        -                                      n                    0                    2                                                  )                                                    ⁢                  xe2x80x83                )            xc3x97              λ                  2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      n            1                                ≤    d    ≤                  arccos        (                                                                              n                  1                  2                                ⁢                                  n                  1                  2                                ⁢                                  a                  4                                            -                              n                0                2                                                                    (                                  1                  +                                      n                    1                    2                                                  )                            ⁢                              (                                                                            n                      1                      2                                        ⁢                                          a                      4                                                        -                                      n                    0                    2                                                  )                                                    ⁢                  xe2x80x83                )            xc3x97              λ                  2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      n            1                                or                    arccos        (                  -                                                                                          n                    1                    2                                    ⁢                                      n                    1                    2                                    ⁢                                      a                    4                                                  -                                  n                  0                  2                                                                              (                                      1                    +                                          n                      1                      2                                                        )                                ⁢                                  (                                                                                    n                        1                        2                                            ⁢                                              a                        4                                                              -                                          n                      0                      2                                                        )                                                                    ⁢                  xe2x80x83                )            xc3x97              λ                  2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      n            1                                ≤    d    ≤                  arccos        (                  -                                                                                                                n                      1                      2                                        16                                    ⁢                                      n                    1                    2                                    ⁢                                      a                    4                                                  -                                  n                  0                  2                                                                              (                                      1                    +                                                                  n                        1                        2                                            16                                                        )                                ⁢                                  (                                                                                    n                        1                        2                                            ⁢                                              a                        4                                                              -                                          n                      0                      2                                                        )                                                                    ⁢                  xe2x80x83                )            xc3x97              λ                  2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      n            1                                    a    =                  (                              n            1                                n            2                          )            k      
is satisfied.
A multi-layer reflection film is constituted of the first to third layers. If the thickness of the third layer is set to the above-described range, the reflectivity of the reflection film when the external medium in contact with the third layer has a first refractive index becomes equal to the reflectivity when it has a second refractive index.
According to another aspect of the present invention, there is provided a laser device comprising: a laser medium having an oscillation wavelength xcex[nm], an effective refractive index n0 and two reflection facets defining an optical resonator; a first layer formed on at least one reflection facet of the laser medium, made of silicon oxide and having a thickness d1 [nm]; a second layer formed on a surface of the first layer, made of silicon having a refractive index nsi and having a thickness d2 [nm]; a third layer formed on a surface of the second layer, made of silicon oxide having a thickness d3 [nm]; wherein the effective refractive index n0 is in a range from 3.18 or larger to 3.28 or smaller, the thickness d1 is in a range of:
(0.11xe2x88x929.2xc3x9710xe2x88x923R+2.2xc3x9710xe2x88x924R2)xcex/1.45xc2x115,
the thickness d2 is in a range of:
(xe2x88x928.7xc3x9710xe2x88x923+3.5xc3x9710xe2x88x923Rxe2x88x921.2xc3x9710xe2x88x925R2)xc3x97(xe2x88x923.6+17/nsi)xcexxc2x115,
and the thickness d3 is in a range of:
(0.23xe2x88x924.9xc3x9710xe2x88x923R+7.7xc3x9710xe2x88x925R2)xcex/1.45xc2x115,
and R [%] is in a range from 15 to 30.
According to another aspect of the present invention, there is provided a method of manufacturing a reflection film comprising the steps of: preparing an optical medium having a reflection surface and a refractive index n0 in a range from 3.18 or larger to 3.28 or smaller; determining a wavelength xcex of light to be reflected and a reflectivity R [%]; forming a first layer on the reflection surface of the optical medium, the first layer being made of silicon oxide and having a thickness d1 [nm] which is in a range of:
(0.11xe2x88x929.2xc3x9710xe2x88x923R+2.2xc3x9710xe2x88x924R2)xcex/1.45xc2x115;
forming a second layer on a surface of the first layer, the second layer being made of silicon having a refractive index ns1 and having a thickness d2 [nm] which is in a range of:
(xe2x88x928.7xc3x9710xe2x88x923+3.5xc3x9710xe2x88x923Rxe2x88x921.2xc3x9710xe2x88x925R2)xc3x97(xe2x88x923.6+17/nsi)xcexxc2x115;
and forming a third layer on a surface of the second layer, the third layer being made of silicon oxide having a thickness d3 [nm] which is in a range of:
(0.23xe2x88x924.9xc3x9710xe2x88x923R+7.7xc3x9710xe2x88x925R2)xcex/1.45xc2x115.
If the thicknesses of the first to third layers are selected so as to satisfy the above equations, a difference between the reflectivity in the atmospheric air and the reflectivity after resin sealing can be made small.
As above, even if the external medium of the reflection film is changed, a change of the reflectivity of light having a specific wavelength can be made small. If this reflection film is formed on a reflection facet of the optical resonator of a laser diode, it is possible to predict the threshold current after resin sealing at a high precision, by measuring the threshold current in the atmospheric air.